A. Field of the Invention
This invention relates generally to machine vision systems and semiconductor chip wire bonding devices, and similar bonding apparatus, and particularly to a method and apparatus for locating and inspecting the bond formed on a lead on a semiconductor chip.
B. Background
Semiconductor devices, such as integrated circuit chips, are electrically connected to leads on a lead frame by a process known as wire bonding. The wire bonding operation involves placing and connecting a wire to electrically connect a pad residing on a die (semiconductor chip) to a lead in a lead frame. Once all the pads and leads on the chip and lead frame have been wire bonded, it can be packaged, often in ceramic or plastic, to form an integrated circuit device. In a typical application, a die or chip may have hundreds or thousands of pads and leads that need to be connected.
There are many types of wire bonding equipment. Some use thermal bonding, some use ultra-sonic bonding and some use a combination of both. A post-process inspection step, commonly called the third optical inspection, typically involves locating and inspecting the position and size of all bonds on the device, the wire connections and the wire heights using optical means to insure that a good connection was created by the bond. This is done after wire bonding is complete and before encapsulation of the integrated circuit.
Heretofore the third optical inspection has been accomplished only after the device is completely bonded and sent to a separate machine or operator, but most usually by a human operator using a microscope. The inspection is often done on a sampled basis. While sampling reduces the number of inspections, bad connections may be missed.
Vision systems or image processing systems (systems that capture images, digitize them and use a computer to perform image analysis) are used on wirebonding machines to align devices and guide the machine for correct bonding placement, but have heretofore not been used during the process for inspection purposes. Separate machines are available to perform inspections outside of and after the bonding process, but this requires another piece of capital equipment in the production line.
Offline machines tend to be slow and may not be as accurate as an in process inspection, since significant information about the device is not available to an offline system. As a result, defective bonds may not be detected until later in the assembly process or after its completion. Productivity is thus lowered, and some defective bonds may not be detected at all.
In actual application in the field, almost all wirebond inspections are done manually. Manual inspection is very slow. Expensive parts may need to be inspected 100% on an individual basis, rather than sampled, and if this is done manually as well, this can further reduce productivity and add to costs.
In order to perform an automated, in-process inspection of the leads bonded by the wire bonder, a system must first locate each soldered lead formed on a chip before an inspection can take place. Applicants' co-pending U.S. applications:
Automated Optical Inspection Apparatus, filed Oct. 6, 1993, Ser. No. 08/132,532; and continuation in part thereof, Automated Optical Inspection Apparatus Using Nearest Neighbor Interpolation, Ser. No. 08/236,215, filed May 2, 1994; which are hereby incorporated by reference, describe a system for locating the nominal placement of a bonded lead with sufficient speed and accuracy to permit an inspection of lead size and position to occur.
Even when the nominal location of a soldered lead can be found quickly, however, one of the major problems with trying to locate and measure bonds on leads in an image are the large number of confusing edges that are present. These are due to the circuitry, probe marks and bond pad or multiple leads & wires in the image. Most semiconductor chips have a considerable amount of visual detail (such as the images of the circuits themselves) which must be circumvented in analyzing the post-bond image to find the bonded lead. In post-process inspections, some of this detail can be mistaken for parts of the bonds. Visual imperfections on the pads caused by probe marks, discoloration, or imperfect illumination further complicate these difficulties.
Wire bonding mechanisms that use a capillary means to affix the wire from the contact pad to the lead on the lead frame usually leave a circular or elliptical indentation at the point where the bond head strikes the lead. This indentation is roughly the diameter of the capillary which forms the end of the bond head and often in the shape of a ring. The formation of the ring is affected by the angle of the wire connecting the die to the lead, the force used in making the bond, the frequency at which the capillary is vibrated, and other factors controlled by the bonding device. While an entire ring is often seen, it is also common for only an arc shaped part of it to appear. In addition, part of the bond known as the crescent, or stitch, sometimes extends into the ring as side effect of the bonding process. This stitch intrusion may obscure a significant portion of the ring or arc indentation.
To bond the wire to the lead, the bond head using the capillary is vibrated at extremely high speed. As it moves on the lead, the indentation is formed. An inner diameter smaller than the chamfer diameter and an outer diameter larger than it appear. Their size depends primarily on the bond head frequency, the shape of the capillary, and the force used in bonding.
The indentation's reflective properties are not always consistent across the indentation. In practice there may be a fringe area immediately beyond the inner and outer diameters where the image intensity decreases (or increases, depending on the light/dark polarity of the image) until it attains the intensity of the lead.
Other problems can be created by mispositioning of the bond head or improper bond force.
If the indentation can be found and measured, its presence, placement, and size indicate the quality of the connection formed on the lead. However, the visual complexity and variability surrounding the lead makes it difficult to automate this inspection, either during an in process operation or as an post-process, offline inspection.